Lattice dynamics and vibrational properties of scheelite-type alkali-metal perrhenates.

The present work provides insight into the structural, vibrational, and elastic properties of scheelite-type alkali-metal perrhenates AReO4 (A = Na, K, Rb, Cs) from first-principles calculations. Sodium, potassium, and rubidium perrhenates are isostructural and crystallize in a tetragonal structure whereas caesium perrhenate crystallizes in an orthorhombic structure. All phonon frequencies and their corresponding mode assignments were estimated, through the linear response method within density-functional-perturbation theory. The phonon density of states highlights the predominant participation of the oxygen anions and both the A-type and rhenium (Re) cations in the low-frequency range. In contrast, oxygen and Re atoms exhibit higher and moderate contributions to the remaining phonon frequency spectrum. Considerable splitting of the longitudinal and traverse optic modes was observed. Elastic constants and phonon dispersion calculations confirm the mechanical and dynamical stability of all the studied AReO4 compounds. A redshift was observed due in the frequency of the phonons following the sequence Na→Cs. The calculated low bulk modulus (ranging from 28.36 GPa to 14.15 GPa) and shear modulus values indicate a low resistance to deformation of the perrhenates. The values of these moduli decrease in the order of Na→Cs which is associated with the increase in the ionic radius of the anion. The response to pressure was found to be anisotropic. This characteristic and the ductile nature of the alkali-metal perrhenates, were confirmed through an elastic analysis. .

Pressure-Sensitive Multiple Superconducting Phases and Their Structural Origin in Van der Waals HfS_{2} Up to 160 GPa.

Superconductivity has been observed in many insulating transition metal dichalcogenides (TMDCs) under pressure. However, the origin of superconductivity remains elusive due to the lack of studies on their structures at low temperatures. Here, we report the observation of a high-T_{c} superconducting state (SC-I phase) coexisting with other superconducting states in a compressed 1T-HfS_{2} crystal up to approximately 160 GPa. In situ synchrotron x-ray diffraction results exclude the presence of decomposed sulfur and confirm two structural phase transitions at room temperature, as well as an additional transition at low temperature, which contribute to the emergence of multiple superconducting states. The SC-I phase exhibits an unsaturated T_{c} of 16.4 K at 158 GPa, and demonstrates the highest upper critical field among the bulk TMDCs, µ_{0}H_{c2}(0)≈29.7 T for a T_{c}∼15.2 K at 147 GPa, exceeding the weak-coupling Pauli limit. These results reveal abundant SC properties together with sensitive structures in compressed HfS_{2}, and thereby extend our understanding on TMDCs' superconductivity.

High-pressure polymorph of Co3P2O8: phase transition to an olivine-related structure.

The monoclinic polymorph of Co3P2O8 (space group P21/c), isomorphic to farringtonite (Mg3P2O8) type orthophosphates, was studied up to 21 GPa using synchrotron powder X-ray diffraction and density-functional theory simulations to investigate the influence of pressure in the crystal structure. This study revealed a pressure induced structural phase transition for monoclinic cobalt phosphate, Co3P2O8, and the details of crystal structure of the new high-pressure polymorph were delineated. The evolution of XRD pattern with pressure indicate that the onset of a phase transition occurs around 2.90(5) GPa, and the low- and high- pressure phases coexist up to 10.3(1) GPa. The high-pressure phase also has a monoclinic lattice (space group P21/c), and a discontinuous change of unit-cell volume of about 6.5% occurs at the transition. A reorganization of atomic positions with a change in the cobalt coordination sphere occurs in the phase transition. Notably, the high-pressure polymorph has a defect-olivine-type structure like chopinite-type orthophosphates. Using a third-order Birch-Murnaghan equation of state, the bulk moduli of the low pressure (LP) phase (75(2) GPa) and high-pressure (HP) phase (92(2) GPa) were determined. For the low-pressure polymorph, the principal axes of compression and their compressibility were also determined. Density-functional theory calculations also provided the frequencies of Raman- and infrared-active modes which can be used for mode assignment in future experiments.

Janus ScYCBr2 MXene as a Promising Thermoelectric Material.

Finding green energy resources that contribute to the battle against global warming and the pollution of our planet is an urgent challenge. Thermoelectric electricity production is a clean and efficient method of producing energy; consequently, scientists are currently researching and creating thermoelectric materials to increase the efficiency of thermoelectric electricity production and expand the potential of the thermoelectric effect for clean energy production. This work focuses on a comprehensive study of the thermoelectric properties of two-dimensional ScYCBr2. We report here a computational analysis of this Janus-like MXene, which is predicted to exhibit outstanding thermoelectric properties. The study uses density-functional theory to provide evidence of the important role played by symmetry breaking to promote low-thermal transport by favoring certain phonon scattering channels. Compared to its symmetric parent compounds, the asymmetric Janus-type ScYCBr2 displays additional phonon scattering channels reducing the thermal conductivity. An exhaustive investigation of the dynamical stability for both zero-temperature and high-temperature conditions was also performed to support the stability of ScYCBr2. Our analysis shows that thanks to its asymmetric structure, the ScYCBr2 MXene has thermoelectric properties that largely surpass those of its parent symmetric counterpart Sc2CBr2, being a material with a remarkable thermoelectric high figure of merit. Another advantage of ScYCBr2 is its high carrier mobility. This work not only demonstrates that this material is a promising thermoelectric material but also shows that ScYCBr2 can operate efficiently at high temperatures up to 1200 K.

Orthorhombic lead-free hybrid perovskite CH3NH3SnI3 under strain: an ab initio study.

We report a computational study where we explore the possibility of tuning the electronic properties of orthorhombic methylammonium tin iodide CH3NH3SnI3 using strains. According to our findings, a moderate [001] strain, smaller than 2%, would open the band gap up to 1.25 eV and enhance the exciton binding energy, opening up new possibilities for the use of CH3NH3SnI3 in technological applications. To better understand the impact of strain, we also examined its influence on bonding properties. The results reveal that the directional pnictogen and the hydrogen bonding are not altered by strains and that the tuning of the electronic properties is the result of changes induced in the orbital contributions to states near the Fermi level and the tilting of the SnI6 octahedral units.

Synthesis, Structural Characterization, and Infrared Analysis of Double Perovskites Pr2NiMnO6, Gd2NiMnO6, and Er2NiMnO6 Functional Nano-Ceramics.

We synthesized Pr2NiMnO6, Gd2NiMnO6, and Er2NiMnO6 double perovskites in a nano-ceramic form by a sol-gel method. By means of room-temperature X-ray powder diffraction measurements, we determined the crystal structure of the three compounds, which is monoclinic, corresponding to a double perovskite structure, described by space group P21/n structure. From the determined structures, the bulk moduli were estimated to be 173-179 GPa. The average size particle of nanoparticles was determined from X-ray diffraction by the Langford method plot and by the Scherrer formula. The morphology and homogeneity of nanoparticles were analyzed by scanning electron microscopy. We found that they form compact agglomerations of approximately 200 nm in diameter. Fourier transform infrared spectroscopy measurements were performed, determining the absorption spectrum. The assignment of the measured infrared absorption bands is discussed.

PbV2O6 under compression: near zero-linear compressibility and pressure-induced change in vanadium coordination.

This study presents evidence that lead metavanadate, PbV2O6, is a material with zero-linear compressibility, which maintains its crystal size in one crystallographic direction even under external pressures of up to 20 GPa. The orthorhombic polymorph of PbV2O6 (space group Pnma) was studied up to 20 GPa using synchrotron powder X-ray diffraction, Raman spectroscopy, and density-functional theory simulations to investigate its structural and vibrational evolution under compression. Up to this pressure we find no evidence of any structural phase transitions by any diagnostic technique, however, a progressive transformation of the coordination polyhedron of vanadium atoms is revealed which results in the zero-linear compressibility. High-pressure Raman experiments enabled the identification and symmetry assignation of all 54 zone-centre Raman-active modes as well as the calculation of their respective pressure coefficients. Three independent high-pressure powder X-ray diffraction experiments were performed using different pressure-transmitting media (Ne, 4 : 1 methanol-ethanol mixture, and silicone oil). The results show a high anisotropic behaviour in the linear compressibility of the crystallographic axes. The PbV2O6 bulk modulus of 86.1(9) GPa was determined using a third-order Birch-Murnaghan equation of state. The experimental results are supported by ab initio density-functional theory calculations, which provide vibrational patterns, unit-cell parameters, and atomic positions. These calculations also reveal that, unlike MgV2O6 and ZnV2O6, the band gap of PbV2O6 closes with pressure at a rate of -54 meV GPa-1 due to the contribution of the Pb 6s orbital to the top of the valence band.

Electronic, Vibrational, and Structural Properties of the Natural Mineral Ferberite (FeWO4): A High-Pressure Study.

This paper reports an experimental high-pressure study of natural mineral ferberite (FeWO4) up to 20 GPa using diamond-anvil cells. First-principles calculations have been used to support and complement the results of the experimental techniques. X-ray diffraction patterns show that FeWO4 crystallizes in the wolframite structure at ambient pressure and is stable over a wide pressure range, as is the case for other wolframite AWO4 (A = Mg, Mn, Co, Ni, Zn, or Cd) compounds. No structural phase transitions were observed for FeWO4, in the pressure range investigated. The bulk modulus (B0 = 136(3) GPa) obtained from the equation of state is very close to the recently reported value for CoWO4 (131(3) GPa). According to our optical absorption measurements, FeWO4 has an indirect band gap that decreases from 2.00(5) eV at ambient pressure to 1.56(5) eV at 16 GPa. First-principles simulations yield three infrared-active phonons, which soften with pressure, in contrast to the Raman-active phonons. These results agree with Raman spectroscopy experiments on FeWO4 and are similar to those previously reported for MgWO4. Our results on FeWO4 are also compared to previous results on other wolframite-type compounds.

Deep Earth Chronicles: High-Pressure Investigation of Phenakite Mineral Be2SiO4.

Beryllium silicate, recognized as the mineral phenakite (Be2SiO4), is a prevalent constituent in Earth's upper mantle. This study employs density-functional theory (DFT) calculations to explore the structural, mechanical, dynamical, thermodynamic, and electronic characteristics of this compound under both ambient and high-pressure conditions. Under ideal conditions, the DFT calculations align closely with experimental findings, confirming the mechanical and dynamical stability of the crystalline structure. Phenakite is characterized as an indirect band gap insulator, possessing an estimated band gap of 7.83 eV. Remarkably, oxygen states make a substantial contribution to both the upper limit of the valence band and the lower limit of the conduction band. We delved into the thermodynamic properties of the compound, including coefficients of thermal expansion, free energy, entropy, heat capacity, and the Gruneisen parameter across different temperatures. Our findings suggest that Be2SiO4 displays an isotropic behavior based on estimated anisotropic factors. Interestingly, our investigation revealed that, under pressure, the compression of phenakite is not significantly affected by bond angle bending.

Tuning the electronic properties of asymmetric YZrCOF MXene for water splitting applications: an ab initio study.

Identifying and evaluating novel and extremely stable materials for catalysis is one of the major challenges that mankind faces today to rapidly reduce the dependence on fossil fuels. To contribute to achieving this goal, we have evaluated within the density-functional framework the properties of a new two-dimensional MXene structure, the asymmetric MXene YZrCOF monolayer. Phonon dispersion calculations at 0 K and 300 K indicate that the studied material is dynamically stable. The calculations also indicate that the material has a rigid crystal structure with a wide band gap, a strong potential difference, and a band-gap alignment that favors the production of both H2 and O2 molecules from water splitting. We also report the outcome of the strain effect on the electrical and photocatalytic characteristics of the studied material. We will demonstrate that even under a large strain, the YZrCOF monolayer is stable and useful for photocatalytic applications.

High-Pressure Behavior of Ca2SnO4, Sr2SnO4, and Zn2SnO4.

The pressure-induced structural evolution of Ca2SnO4, Sr2SnO4, and Zn2SnO4 has been characterized by powder X-ray diffraction up to 20 GPa using the ALBA synchrotron radiation source and density functional theory calculations. No phase transition was observed in Ca2SnO4 and Zn2SnO4 in the investigated pressure range. The observation in Zn2SnO4 solves contradictions existing in the literature. In contrast, a phase transition was observed in Sr2SnO4 at a pressure of 9.09 GPa. The transition was characterized as from the ambient-condition tetragonal polymorph (space group I4/mmm) to the low-temperature tetragonal polymorph (space group P42/ncm). The linear compressibility of crystallographic axes and room-temperature pressure-volume equation of state are reported for the three compounds studied. Calculated elastic constants and moduli are also reported as well as a systematic discussion of the high-pressure behavior and bulk modulus of M2SnO4 stannates.

Joint experimental and theoretical study of PbGa2S4 under compression.

The effect of pressure on the structural, vibrational, and optical properties of lead thiogallate, PbGa2S4, crystallizing under room conditions in the orthorhombic EuGa2S4-type structure (space group Fddd), is investigated. The results from X-ray diffraction, Raman scattering, and optical-absorption measurements at a high pressure beyond 20 GPa are reported and compared not only to ab initio calculations, but also to the related compounds α'-Ga2S3, CdGa2S4, and HgGa2S4. Evidence of a partially reversible pressure-induced decomposition of PbGa2S4 into a mixture of Pb6Ga10S21 and Ga2S3 above 15 GPa is reported. Thus, our measurements and calculations show a route for the high-pressure synthesis of Pb6Ga10S21, which is isostructural to the stable Pb6In10S21 compound at room pressure.

Exploring the crystal structure and properties of ytterbium orthoantimonate under high pressure.

The crystal structure of YbSbO4 was determined from powder X-ray diffraction data using the Rietveld method. YbSbO4 is found to be monoclinic and isostructural to α-PrSbO4. We have also tested the influence of pressure on the crystal structure up to 22 GPa by synchrotron powder X-ray diffraction. No phase transition was found. The P-V equation of state and axial compressibilities were determined. Experiments were combined with density-functional theory calculations, which provided information on the elastic constants and the influence of pressure in the crystal structure and Raman/infrared phonons. Results are compared with those from other orthoantimonates. Reasons for the difference in the high-pressure behaviour of YbSbO4 compared with most antimony oxides will be discussed.

Theoretical study of electrocatalytic properties of low-dimensional freestanding PbTiO3 for hydrogen evolution reactions.

The discovery of novel materials for catalytic purposes that are highly stable is one of the main challenges nowadays for reducing our dependence on fossil fuels. Here, low-dimensional PbTiO3 is introduced as an electrocatalyst using first-principles calculations. Density-functional theory calculations indicate that 2D-PbTiO3 is dynamically and thermodynamically stable. Our results show that a single oxygen defect vacancy in 2D-PbTiO3 can play a key role in enhancing the hydrogen evolution reaction (HER), together with the Ti atoms. Our study concludes that the Volmer-Heyrovsky mechanism is a more favorable route to achieve HER than the Volmer-Tafel mechanism, including solvation and vacuum conditions.

MYB24 orchestrates terpene and flavonol metabolism as light responses to anthocyanin depletion in variegated grape berries.

Variegation is a rare type of mosaicism not fully studied in plants, especially fruits. We examined red and white sections of grape (Vitis vinifera cv. 'Béquignol') variegated berries and found that accumulation of products from branches of the phenylpropanoid and isoprenoid pathways showed an opposite tendency. Light-responsive flavonol and monoterpene levels increased in anthocyanin-depleted areas in correlation with increasing MYB24 expression. Cistrome analysis suggested that MYB24 binds to the promoters of 22 terpene synthase (TPS) genes, as well as 32 photosynthesis/light-related genes, including carotenoid pathway members, the flavonol regulator HY5 HOMOLOGUE (HYH), and other radiation response genes. Indeed, TPS35, TPS09, the carotenoid isomerase gene CRTISO2, and HYH were activated in the presence of MYB24 and MYC2. We suggest that MYB24 modulates ultraviolet and high-intensity visible light stress responses that include terpene and flavonol synthesis and potentially affects carotenoids. The MYB24 regulatory network is developmentally triggered after the onset of berry ripening, while the absence of anthocyanin sunscreens accelerates its activation, likely in a dose-dependent manner due to increased radiation exposure. Anthocyanins and flavonols in variegated berry skins act as effective sunscreens but for different wavelength ranges. The expression patterns of stress marker genes in red and white sections of 'Béquignol' berries strongly suggest that MYB24 promotes light stress amelioration but only partly succeeds during late ripening.

Band-Gap Energy and Electronic d-d Transitions of NiWO4 Studied under High-Pressure Conditions.

We report an extensive study of the optical and structural properties of NiWO4 combining experiments and density functional theory calculations. We have obtained accurate information on the pressure effect on the crystal structure determining the equation of state and compressibility tensor. We have also determined the pressure dependence of the band gap finding that it decreases under compression because of the contribution of Ni 3d states to the top of the valence band. We report on the sub-band-gap optical spectrum of NiWO4 showing that the five bands observed at 0.95, 1.48, 1.70, 2.40, and 2.70 eV correspond to crystal-field transitions within the 3d8 (t2g6eg2) configuration of Ni2+. Their assignment, which remained controversial until now, has been resolved mainly by their pressure shifts. In addition to the transition energies, their pressure derivatives are different in each band, allowing a clear band assignment. To conclude, we report resistivity and Hall-effect measurements showing that NiWO4 is a p-type semiconductor with a resistivity that decreases as pressure increases.

Pressure-Induced Phase Transition versus Amorphization in Hybrid Methylammonium Lead Bromide Perovskite.

The crystal structure of the CH3NH3PbBr3 perovskite has been investigated under high-pressure conditions by synchrotron-based powder X-ray diffraction. We found that after the previously reported phase transitions in CH3NH3PbBr3 (Pm3̅m→Im3̅→Pmn21), which occur below 2 GPa, there is a third transition to a crystalline phase at 4.6 GPa. This transition is reported here for the first time contradicting previous studies, which reported amorphization of CH3NH3PbBr3 between 2.3 and 4.6 GPa. Our X-ray diffraction measurements show that CH3NH3PbBr3 remains crystalline up to at least 7.6 GPa, the highest pressure covered by experiments. The new high-pressure phase is also described by the space group Pmn21; however, the transition involves abrupt changes in the unit-cell parameters and a 3% decrease of the unit-cell volume. Our conclusions are confirmed by optical-absorption experiments, by visual observations, and by the fact that pressure-induced changes up to 10 GPa are reversible. The optical studies also allow for the determination of the pressure dependence of the band-gap energy, which is discussed using the structural information obtained from X-ray diffraction.

2D Cu(I)-I Coordination Polymer with Smart Optoelectronic Properties and Photocatalytic Activity as a Versatile Multifunctional Material.

This work presents two isostructural Cu(I)-I 2-fluoropyrazine (Fpyz) luminescent and semiconducting 2D coordination polymers (CPs). Hydrothermal synthesis allows the growth of P-1 space group single crystals, whereas solvent-free synthesis produces polycrystals. Via recrystallization in acetonitrile, P21 space group single crystals are obtained. Both show a reversible luminescent response to temperature and pressure. Structure determination by single-crystal X-ray diffraction at 200 and 100 K allows us to understand their response as a function of temperature. Applying hydrostatic/uniaxial pressure or grinding also generates significant variations in their emission. The high structural flexibility of the Cu(I)-I chain is significantly linked to the corresponding alterations in structure. Remarkably, pressure can increase the conductivity by up to 3 orders of magnitude. Variations in resistivity are consistent with changes in the band gap energy. The experimental results are in agreement with the DFT calculations. These properties may allow the use of these CPs as optical pressure or temperature sensors. In addition, their behavior as a heterogeneous photocatalyst of persistent organic dyes has also been investigated.

Equations of State and Crystal Structures of KCaPO4, KSrPO4, and K2Ce(PO4)2 under High Pressure: Discovery of a New Polymorph of KCaPO4.

We have studied by means of angle-dispersive powder synchrotron X-ray diffraction the structural behavior of KCaPO4, SrKPO4, and K2Ce(PO4)2 under high pressure up to 26, 25, and 22 GPa, respectively. For KCaPO4, we have also accurately determined the crystal structure under ambient conditions, which differs from the structure previously reported. Arguments supporting our structural determination will be discussed. We have found that KCaPO4 undergoes a reversible phase transition. The onset of the transition is at 5.6 GPa. It involves a symmetry decrease. The low-pressure phase is described by space group P3̅m1 and the high-pressure phase by space group Pnma. For KSrPO4 and K2Ce(PO4)2, no evidence of phase transitions has been found up to the highest pressure covered by the experiments. For the three compounds, the linear compressibility for the different crystallographic axes and the pressure-volume equation of states are reported and compared with those of other phosphates. The three studied compounds are among the most compressible phosphates. The results of the study improve the knowledge about the high-pressure behavior of complex phosphates.

Accurate Determination of the Bandgap Energy of the Rare-Earth Niobate Series.

We report diffuse reflectivity measurements in InNbO4, ScNbO4, YNbO4, and eight rare-earth niobates. A comparison with established values of the bandgap of InNbO4 and ScNbO4 shows that Tauc plot analysis gives erroneous estimates of the bandgap energy. Conversely, accurate results are obtained considering excitonic contributions using the Elliot-Toyozawa model. The bandgaps are 3.25 eV for CeNbO4, 4.35 eV for LaNbO4, 4.5 eV for YNbO4, and 4.73-4.93 eV for SmNbO4, EuNbO4, GdNbO4, DyNbO4, HoNbO4, and YbNbO4. The fact that the bandgap energy is affected little by the rare-earth substitution from SmNbO4 to YbNbO4 and the fact that they have the largest bandgap are a consequence of the fact that the band structure near the Fermi level originates mainly from Nb 4d and O 2p orbitals. YNbO4, CeVO4, and LaNbO4 have smaller bandgaps because of the contribution from rare-earth atom 4d, 5d, or 4f orbitals to the states near the Fermi level.